import UIKit

let words = ["Cat", "Chicken", "Fish", "Dog", "Mouse", "Pig", "Monkey"]

typealias Entry = (Character, [String])

func buildIndex(words: [String]) -> [Entry] {
    var result = [Entry]()
    var letters = [Character]()

    for word in words {
        let firstLetter = Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString)

        if !contains(letters, firstLetter) {
            letters.append(firstLetter)
        }
    }

    for letter in letters {
        var wordsForLetter = [String]()
        for word in words {
            let firstLetter = Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString)

            if firstLetter == letter {
                wordsForLetter.append(word)
            }
        }
        result.append((letter, wordsForLetter))
    }
    return result
}

println(buildIndex(words))